The rotation speed of a three-phase alternating current (AC) motor depends upon the frequency of the AC signal input into the motor. Where it is desirable to control the speed of such a motor, a driver circuit is commonly used to convert the original AC power source signal (which is typically fixed) to a desired driving frequency and/or voltage. The driver circuit may also include a regeneration capability, which receives energy back from the motor, such as when the motor winds down or resists an external force attempting to increase the speed of the motor beyond a controlled speed.
Such driver circuits typically include a pair of three-phase inverters, one for driving the motor and the other for performing the regeneration function. Where a transformer is used between the power source and the driver circuit to obtain a higher drive voltage, then there is typically one three-phase converter and one single-phase inverter for each motor phase. Moreover, the transformer will include multiple secondary isolation windings. The end result is often a complex and costly system, utilizing many devices and voltage/current detection circuits.